An "alternative" G8 summit is to be organised by protesters against global austerity measures.

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Trade unionists, anti-war activists, charities, anti-fracking organisations, environmentalists and pro-Palestinian lobbyists will be involved in demonstrations to be held near this summer's conference in Northern Ireland as well as in Belfast, Dublin and London, one of the organisers said.

Police are mounting a massive security operation in anticipation of trouble including creating extra cells capable of holding 350 people and imposing a ring of steel around the luxury Lough Erne golf resort in Co Fermanagh where June's meeting of world leaders is being held.

US President Barack Obama and Russian President Vladimir Putin are among those expected to attend a gathering of leaders representing some of the world's leading economies hosted by British Prime Minister David Cameron.

People Before Profit campaigner Eamonn McCann said: "Here we have these eight people who are all in one way or another representing big power politics, they are all in one way or another involved in imposing austerity. They represent the 1% against the 99%."

The Police Service of Northern Ireland (PSNI) is expanding custody provision at stations across Northern Ireland for the summit. Courts and prisons staff will also step up workloads in the anticipation of extra arrests. An unused 108-capacity unit at Maghaberry high security prison in Co Antrim and a former army site in Co Tyrone could house those charged.

Mr McCann accused the media of focusing on a violent minority and said past G8 demonstrators had been angered by the sight of US security forces on the streets. "I would hope that the protests would be peaceful but you cannot rule out provocation in some form," he said.

"The main focus of the violence in 2005 (Gleneagles) was the tactics by foreign security services. In Scotland groups of people were being confronted by American security agents who had taken over control."

He said it was remarkable that the PSNI had been granted late-night sittings of courts to charge suspected offenders and demanded magistrates be made available on a Sunday without any opposition from local politicians. "Where is the democratic accountability of law and order in all this?" he asked.

Expected participants in protests include the War on Want and Trocaire aid charities, groups concerned about fracking for gas (Fermanagh is a potential site for the mining), organisations supporting Palestinian causes and Friends of the Earth.